It is known that a piezoelectric ceramic-polymer composite can be produced by mixing ferroelectric ceramic particles of lead zirco-titanate or lead titanate with piezoelectric organic material such as polyvinylidene fluoride, polyvinyl fluoride, polyvinylidene chloride, polyvinyl chloride, nylon or organic material such as synthetic rubber or synthetic resin, and has acoustic impedance characteristics similar to the acoustic impedance of water as compared with a conventional piezoelectric ceramic material. It is also known that when such a composite material is used for a piezoelectric transducer, it can efficiently receive acoustic waves propagated under water to provide advantages of increasing the sensitivity. A piezoelectric transducer of this type is described in U.S. Pat. Nos. 3,921,125, 3,970,862, 4,183,010, 4,056,742 and 4,376,302.
U.S. Pat. No. 4,183,010 discloses a coaxial line hydrophone comprising radially spaced conductors and intermediate dielectric filler material that has both piezoelectric and electret properties such as polyvinylidene fluoride, and method of measuring underwater soundings at either low or high hydrostatic pressures by measuring electrostatically generated signals between the conductors at low hydrostatic pressures and piezoeletrically generated signals at high hydrostatic pressures.
U.S. Pat. No. 4,376,302 discloses a piezoelectric polymer hydrophone in which it comprises a single flexible sheet of a piezoelectric polymer having a plurality of electrode strips on the top and bottom thereof, the electrode strips at the top are staggered by one half the width of a strip relative to the corresponding strips at the bottom of the sheet and the polymer sheet can be rolled into a helix without losing its acoustic sensitivity.
In the prior art as mentioned above, since the piezoelectric device is flexible, the piezoelectric portion of the device is subjected to not only an acoustic wave but also a pressure such as, for example, tensile stress at the blowing stream, or a bending stress due to the stream of water or wave. By these mechanical stresses is occurred a distortion in the longitudinal direction of the piezoelectric portion, that is, in a direction perpendicular to the polarized axis to generate a charge or voltage which produces a noise signal to be superposed on the acoustic wave, thereby decreasing the S/N ratio.
It is, therefore, an object of the present invention to provide an underwater piezoelectric arrangement capable of eliminating the abovementioned drawbacks and disadvantages of the conventional piezoelectric device by obviating the influence of the mechanical pressure except an acoustic wave.
Another object of the invention is to provide an underwater piezoelectric cable which comprises said piezoelectric arrangement.
A further object of the invention is to provide a piezoelectric arrangement for determining an acoustic wave which is propagating under water.